Friction between a drill string or coiled tubing lowered into an open hole (uncased wellbore) or cased wellbore is a common problem in highly deviated or complex wells, such as horizontal wells, extended wells, and multi-lateral wells, which are formed using directional drilling techniques. The resulting drag impedes movement in and out of the hole of the pipe, as well as, in the case of drill strings, rotation of the drill string, especially once the drill string or coiled tubing stops moving and static friction takes over. When drilling a wellbore, the friction also affects the rate of penetration (ROP) of the drill bit. The full amount of the weight that the drilling operator is trying to put on the bit (the “weight on bit”) is not being transferred to the bit when there is drag.
A “drill string” refers usually to the combination of jointed drill pipe, a drill bit, and other tools that is rotated from the surface to drill through subterranean rock formations to establish a wellbore for recovering deposits of oil and gas from the rock. However, coiled tubing can be used instead of jointed drill pipe to make up a drill string. In either case, drilling fluid or “mud” is pumped through the drill string under high pressure and then circulated back up to the surface through the annulus formed between the drill string and sides of the wellbore after it exits the face of the drill bit. The drilling fluid acts as a medium for evacuating rock cuttings. When a positive displacement or “mud” motor is placed within the drill string, the flow of drilling fluid also powers the mud motor.
Coiled tubing, which is a continuous pipe stored on reels that can be quickly moved in and out of wellbores, can also be used for different applications, such fishing operations, clean outs, operating downhole equipment (such as shiftable sleeves) and in other types of completion and work over operations. Both types of uses of coiled tubing can suffer from the problems associated with friction noted above. A reference to “drill string” is therefore intended to include drill strings that use jointed pipe or coiled tubing for drilling, as well as use of coiled tubing in other applications involving highly deviated or complex wells.
To reduce the effects of friction specialized downhole tools are inserted into the drill string for vibrating it. One well-known example of such a tool is the Agitator™ sold by NOV. Another example is “The Toe Tapper™” from CT Energy. Although some of these types of tools generate lateral and torsional vibrations, most generate axial oscillations in the drill string. The vibrations in the drill string help to reduce the effects of friction by generating cyclical pressure waves within the drilling fluid. Examples of these types of downhole tools are disclosed in U.S. Pat. Nos. 6,237,701, 6,431,294, 8,162,078 and 9,222,312, and U.S. published patent application number 2017/0191325.